For the wrapping of various pieces, for example coils, wrapping devices known as such are used to perform the wrapping by means of a wrapping film, preferably a thin and transparent stretch film made of plastic. The film roll is normally fitted in film feeding means, which are further placed in a carriage which moves along a circular structure and a track formed therein, either around the whole piece or passing via an opening in the central line of the piece.
However, such devices have the problem that the quantity of film to be supplied from the film roll at each moment varies at different locations on the circular tract, because the carriage is not continuously at the same distance from the piece. At some points, the distance is even reduced, having the result that the film between the piece and the carriage does not remain sufficiently stretched all the time, thereby leading to poor wrapping quality, causing tangling of the film or other problems.
One wrapping device is disclosed in EP 0 936 142 A2, or corresponding U.S. Pat. No. 6,192,653. The device comprises a roll device for guiding a film, having a roll placed against the film to maintain the tension of the film but being still allowed to rotate with the film in the feeding direction of the film. The roll is coupled by means of a moment limiter to a motor device which tends to rotate the roll in the opposite direction, but the tension of the film exceeds the set moment and the roll can thus not revolve in the opposite direction. If the film is slackened, the roll can revolve in the opposite direction and the film is re-wound on the film roll, wherein said problem is eliminated. However, problems may be involved in the synchronization of the operation of the guide roll and the film roll, because the film roll may also be provided with a moment limiter.
Another device is presented in EP 0 936 141 B1, in which the tension of the film is continuously monitored with sensor means and in which the film roll can be rotated by means of a motor. If the film is found to become slackened, the film roll is rotated backwards and the loose part is rewound onto the film roll. The device requires an auxiliary motor for the film roll, which is difficult to place in the carriage and which considerably increases the weight of the carriage.
One known device is presented in the publication EP 0 544 312 B1, or corresponding U.S. Pat. No. 5,282,347. The device comprises a roll device placed in a carriage and in which the loose film can be accumulated, if necessary. Some of the rolls are moved by spring force and controlled by changes in the tension of the film. The rolls require a lot of space and increase the weight of the carriage. An increase in the weight will make the moving of the carriage more difficult or will involve an increased power requirement and problems in the placement of the powerful motors.